1. Field of the Invention
The present invention relates to a slidable polyimide film, having fine protrusions on the film surface and a satisfactory film surface sliding property, which is easily wound up onto winding rolls in large lengths and exhibits a heat fusion bonding property, as well as to a copper-clad laminate employing it and, particularly, it relates to an easily slidable polyimide film suitable as a stock film for formation of fine patterns on flexible printed boards, TABs or the like, and to a copper-clad laminate employing it.
2. Description of the Related Art
Polyimide films have been used for laminates, flexible printed boards and the like because of their excellent heat resistance, chemical resistance, mechanical strength and electrical properties. However, polyimide films have been problematic in terms of adhesive properties. Methods for improving the adhesive properties include methods of surface treatment such as alkali treatment, corona treatment, sand blasting and cold plasma treatment. However, while such methods are effective at improving the adhesive properties, they require adhesive agents such as epoxy resin-based adhesive agents to be added in addition to the polyimides, and these lower the overall heat resistance of the flexible substrates.
As a result there have been proposed polyimide films having a thermoplastic polyimide layer, such as a thermoplastic single layer polyimide film or a multilayer polyimide film having a thermoplastic thin-layer prepared by laminating thermoplastic polyimide thin-layers on both sides of a heat-resistant polyimide layer. (These may hereinafter be referred to simply as “thermoplastic polyimide films.)
However, when such thermoplastic polyimide films have smooth surfaces, they experience high friction during production of the wound film on a winding roll, or during lamination with copper films, leading to problems such as creasing or sticking to rolls and, as this has constituted a restriction on winding, a need has existed for improvement in the surface sliding property of thermoplastic polyimide films.
Conventional methods for improving the surface sliding property of polyimide films include methods of surface treatment such as embossing, and methods of reducing surface friction by mixing inorganic powders such as calcium phosphate (Japanese Unexamined Patent Publication No. 62-68852) or silica (Japanese Unexamined Patent Publication No. 62-68853) with the polyimide film to produce fine protrusions on the film surface. There has also been proposed a method of producing a polyimide film by cast film formation of a polyamic acid solution obtained by dispersing a fine inorganic filler for polymerization in the solvent (Japanese Unexamined Patent Publication No. 6-145378).
However, the first type of surface treatment method has had a drawback in that the appearance of the film is easily impaired by excessive irregularities produced in the film surface. The second type of method involves mixing an inorganic powder in a polyamic acid solution, but unless a special dispersing apparatus is used, it is difficult to uniformly disperse the inorganic powder in the polyamic acid solution, and the resulting lack of uniform dispersion produces residual aggregates which often form excessively large protrusions on the film surface. In the third type of method as well, it is difficult to uniformly disperse the fine particulate inorganic powder, and using an inorganic powder with a large particle size leads to a similar problem as in the second type of method.
Consequently, when such inorganic filler addition methods are applied for copper-clad laminates used for COFs or TABs which require fine patterns, the raised sections of the thermoplastic polyimide film surface often creates a hindrance to fine pitch formation.